It is well known that an electron carries not only a quantum of charge, but also a quantum of spin angular momentum, and, thus, electrical currents can transport spin. The utilization of this spin degree of freedom in electronics is tempting, but not trivial because spin-flip scattering in a conducting medium can result in the rapid loss of spin information. The characteristic length scale over which spin information is erased in metals typically does not exceed one micrometer, making the practical use of spin a significant technological challenge. However, recent advances in nanofabrication put spintronic devices with new functionalities within reach [1].
One such putative spintronic device is a microwave voltage-controlled oscillator based on the spin-transfer effect in nanoscale spin valves [1] and nanocontacts to magnetic multilayers [2] FIG. 1(a) shows a nanoscale spin valve having a diameter on the order of 100 nm. When current, I, flows perpendicular to the ferromagnetic layers (Co), a thicker (fixed) ferromagnetic layer serves as a current polarizer and spin-polarized current is injected into the non-magnetic spacer layer (Cu). This spin-polarized current then exerts spin torque on magnetization of the thinner (free) ferromagnetic layer and excites magnetization precession in this layer.
FIG. 1(b) shows a nanocontact to a magnetic multilayer having a diameter on the order of 40 nm. Spin-polarized current injected through the nanocontact excites oscillations of magnetization of the free layer directly under the contact. These oscillations generate spin waves in the free layer that propagate away from the contact. In these multilayer devices, current flows perpendicular to the layers and current densities as high as ˜109 A/cm2 can be achieved.
A thicker (fixed) ferromagnetic layer in these devices serves as a current spin polarizer. When the polarized current enters the thinner (free) layer, it transfers angular momentum to the free layer and thereby exerts torque (called spin torque) on its magnetization [3]. At large enough current densities, the magnitude of the spin torque becomes comparable to the torque caused by the natural magnetic dissipation in the material of the free layer, and the total effective dissipation of magnetic energy becomes negative. This negative effective dissipation leads to the dynamic instability of magnetization of the free layer. Depending on the magnitude of the external bias magnetic field, this instability can lead to either magnetization reversal [1] or to persistent auto-oscillations of magnetization at GHz frequencies [2, 4].
The auto-oscillations of magnetization of the free layer result in a microwave voltage generated by the current-biased device via the giant magnetoresistance (GMR) effect. The frequency of the generated microwave signal was shown to be a strong function of the bias current and the magnitude and direction of the bias magnetic field [2, 4, 5], which makes the spin torque devices promising candidates for nanoscale tunable microwave oscillators.
The development of practical microwave devices based on the spin-torque effect requires, however, a detailed understanding of a number of fundamental properties of the current-driven excitations of magnetization. In particular, at present there are no direct experimental measurements of the spatio-temporal profile of the magnetic excitations driven by spin current and, thus, the question of the nature of the experimentally observed current-driven excitations remains open.
The magnitude of the microwave signals generated by spin transfer devices undoubtedly points to a very large amplitude of the current-driven spin waves [5]. This implies that these spin waves are of a highly nonlinear nature, and it is not clear that standard equations of magnetization dynamics that successfully describe spin waves in linear and moderately nonlinear regimes can give quantitative description of the spin torque excitations. Therefore, the development of spin torque microwave devices requires direct experimental measurements of the properties of current-driven nonlinear spin waves, such as propagation velocity, wavelength, and attenuation as well as development of the comprehensive theory of spin wave excitation by spin-polarized current.